In today's wireless communication networks, Public Land Mobile Networks (PLMNs) have been established to provide land mobile telecommunications service to user devices such as electronic book readers, cellular telephones, personal digital assistants (PDAs), portable media players, tablet computers, netbooks, and the like. Generally, each service provider operates its own PLMN. A user device typically attempts to register with its own service provider's PLMN (the home PLMN) upon initial power up. If the user device is located outside of the area covered by the home PLMN, the user device attempts service acquisition on another PLMN, which is commonly known as roaming. Typically, a prioritized list of PLMNs that the user device is allowed to access is stored on a Subscriber Identity Module (SIM) card. The entries and prioritization in the list are usually based on agreements contracted among the various service providers.
When a user device is camped on a PLMN that is not its home PLMN or the highest priority PLMN, the user device usually performs a PLMN scan trying to find its home PLMN or a higher priority PLMN. A PLMN scan is repeated at predetermined time intervals, as defined in a standard specification, such as the 3rd Generation Partnership Project (3GPP) specification. For example, the 3GGP specification requires that a PLMN scan be performed every (6*n) minutes, where “n” is configurable by a service provider and is often set to 1, resulting in a PLMN scan being performed every 6 minutes.
The PLMN scan procedure requires a user device receiver to be programmed to different frequencies in order to perform power measurements on various frequencies. Hence, in current wireless technology standards supporting second generation (2G) communication systems (e.g., Global Systems for Mobile Communications (GSM)) and third generation (3G) communication systems (e.g., Universal Mobile Telecommunications Systems (UMTS)), periodic PLMN scans are not performed by a user device when the user device is in connected state (i.e., when the user device's receiver is always busy listening to a serving cell). Rather, periodic PLMN scans are performed only when the user device is in idle state (i.e., when the user device has no active radio bearer connection with the network). Often, when the user device is in idle state, the network configures the user device to a discontinuous reception (DRX) mode where the user device wakes up for decoding pages at fixed intervals specified by the network to save power consumption. The user device then usually performs periodic PLMN scans between these DRX wakeups.
In current 2G and 3G systems, user devices are left in idle state most of the time unless they have to send data or make a voice call. This approach provides battery saving benefits but results in longer connection times because a user device needs to establish a radio bearer every time it needs to move from idle to connected state. 3GPP Long Term Evaluation (LTE), the latest wireless technology standard developed to support fourth generation (4G) communication systems, addresses this problem by introducing the DRX concept in connected state and keeping user devices in connected state most of the time to shorten connection times. This, however, makes it difficult for a user device to perform a home or higher priority PLMN scan when the user device is connected to a roaming network.